1. Technical Field
The present invention relates to electronic compasses, and more specifically to a method for compensating for excess vehicle magnetic fields in an electronic compass.
2. Discussion
The present invention is related to and is an improvement of U.S. Pat. No. 4,622,843 to Hormel issued Nov. 18, 1986 and entitled "Simplified Calibration Technique and Auto Ranging Circuit for an Electronic Compass Control Circuit". The present invention is also related to U.S. Pat. No. 4,807,462 issued Feb. 28, 1989, to Rafi A. Al-Attar and entitled "Method for Performing Automatic Calibration in an Electronic Compass." The disclosures of these patents are hereby incorporated by reference.
The present invention is related to and combinable with the commonly assigned patent application "Scaling System and method for an Electronic Compass" Ser. No. 07/815,347. This application is hereby incorporated by reference.
Normally, electronic compass systems employ a magnetic flux-gate sensor. The operation of the flux-gate sensor is well documented. See "Magnetic Field Sensor and Its Application to Automobiles", SAE Paper No. 800123, pages 83-90, February, 1980, by Hisatsugu Itoh; and "A Magnetic Heading Reference for the Electro/Fluidic Autopilot" Sport Aviation by Doug Garner, Part I, pages 19-26, November, 1981 and Part II, pages 20-32, 51, December, 1981. The disclosures of these documents are also hereby incorporated by reference.
Electronic compasses are capable of processing ordinary vehicle and earth magnetic fields. In the '843 patent to Hormel, the output of an integrator is summed with the output of a ranging circuit, the sum is fed back to the flux-gate sensor. The feedback current reduces the output voltage of the flux-gate sensor until the microcomputer senses that the feedback current generates a magnetic field that is equal in magnitude but opposite in direction to the field sensed by the sense coils due to the external measured magnetic field (the earth and vehicle magnetic fields). The microcomputer employs a ranging technique found in commonly assigned U.S. Pat. No. 4,750,349 issued Jun. 14, 1988, to Luitje, entitled "Microcomputer Controlled Quick Ranging Technique and Digital Filter." The disclosure of this patent is also hereby incorporated by reference.
However, the primary purpose of the ranging circuit is to bring the output of the integrator into a range that can be resolved clearly by the A/D which then can feed this information to the microcomputer to handle. Therefore, the ranging circuit is not capable of compensating for abnormally large vehicle magnetic fields, making the geometric method of calibration such as the one found in the '462 patent difficult to perform, where resolving power is defined as the ability of the analog-to-digital converter to partition an analog voltage signal into distinct voltage ranges and assign a different digitized value to each ranges. The feedback current is further limited by the size of the feedback resistors, and the inherent limitations of the ranging circuit.